This invention concerns joints formed at the interface between adjacent bodies of different materials and, more particularly, although not exclusively to a joint formed with a body of a cured or curable material.
For a number of engineering applications, there is a desire to use composite materials due to the material properties and potential weight savings they can offer. However for many applications it is desirable to form products or components only partly of composite material, whereby the composite material is attached to an adjacent body of different material. The combined material properties may thus be tailored to suit engineering requirements. For example the adjacent material may be provided to improve the strength, temperature resistance or other physical or thermal properties of the final product. In this regard, there is a need to securely join a composite structure to a metallic structure.
Such a need has been identified in aerospace applications, for example with respect to fluid washed bodies, e.g. rotor blades, stator vanes, aircraft wings or the like, although the invention is not limited to such applications.
Previous attempts to address this need include the use of fasteners. Some types of fastener, such as bolts, are designed to pass through apertures in the composite and metallic structures and thereby span the interface there-between. These are disadvantageous as the apertures/bolts interrupt the structure of the composite and metallic materials, thereby weakening the structure and/or providing a site at which stress concentrations can occur. Impacts can cause high stress gradients to occur in the vicinity of the apertures. This can potentially cause damage, particularly to the composite structure.
Also the relatively large thermal mass of the bolts can adversely affect the product upon repeated heating and cooling cycles if the product is intended for use in a hot environment. This can weaken or otherwise reduce the thermal life of the product. Other types of fastener, such as H-section fasteners, are located externally to the composite and metallic structures. However any such fasteners are generally disadvantageous as they increase part count, complexity of assembly and may be subject to the stress concentration problems discussed above. Bolts and other fasteners also have a protruding head formation to hold the fastener in place. Such protrusions are typically unwanted, and can significantly impact aerodynamic efficiency if the product is intended to be fluid-washed in use.
Attempts to address this problem include the use of an adhesive layer between the composite and metallic structures. It is awkward to assemble the composite structure to the metallic structure when adhesive is used. Furthermore this method introduces another material component at the interface, potentially enhancing the problems associated with the material discontinuity.
Debonding, or other detachment, of the composite is a problem due to stress applied at the interface between materials of differing properties. For an adhesive bond, the strength of the interface is dependent on the lowest of: the strength of the composite material; the bond between the metal and adhesive; the bond between the adhesive and the composite; and the adhesive itself. The strength is further reduced by the typically linear nature of the joint which is needed in order to ensure adequate adhesive contact area and application. Thus debonding can propagate along the joint, thereby reducing its strength, impact resistance, vibration resistance and operational life.
There has now been devised an improved joint which overcomes or substantially mitigates some or all of the above-mentioned and/or other disadvantages associated with the prior art.